The present invention relates to improvements in apparatus for processing wood pulp and other fibrous fluid suspensions, and methods for manufacturing the apparatus. More particularly, the apparatus and methods relate to an improved screen for wood pulp, for removing foreign particles from a pulp slurry.
In processing wood pulp, screens are utilized to separate acceptable fiber from unacceptable constituents in a slurry. In a typical screen, the slurry flows through a perforate, cylindrical screen plate, which may be smooth, or which may present a contoured surface toward the stock flowing through the screen, to increase the effective screening area. The screen plate openings are formed in different hole or hole and slot combinations for optimizing screening performance. To aid in passage of the acceptable pulp through the screen plate, and to avoid plugging, pulsations are generated in the slurry such as by passing a hydrofoil-shaped member past the screen plate. In order to give the screen plate strength to withstand the pressure differential across the surface, and to increase the screening capacity by presenting increased screening area, it has generally been the practice to provide a thickly-walled screen plate which is machined to present the desired surface. Machining the desired contour has required a time consuming and expensive process. Because of manufacturing restrictions in the machining process imposed at least in part by the machine tools themselves, total available open accepts flow area has been limited in known screen plates, and the final shape of the screen plate has been a compromise between the limitations of machining and the desired optimum screen shape.
In addition to the expensive costs of production and manufacturing, the type of screen described has been expensive to use and maintain in that, even if only a small area of screen is damaged, the entire screen plate, which includes the screening surface, mounting surfaces and support members must be replaced, thereby presenting a costly operating expense.
An additional problem encountered in operating screens using known screen plates is premature wear due to contaminants. In recycling waste paper, contaminants such as metals, sand, plastic, and glass are often present, and screen plates utilized to remove these contaminants experience rapid wear. In some instances, screen plates have been known to last less than seven days before failure has occurred. When heretofore known screen plates are used in such screens, the cost and time required for screen plate replacement is significant.
It is, accordingly, an object of the present invention to provide a screening apparatus and a screen plate design wherein the necessity of an expensive machining manufacturing process is eliminated.
A further object of the present invention is to provide a screen plate structure wherein various modifications and alternatives of screen plate shape can be attained without prohibitive manufacturing costs, and wherein contours can be utilized which were heretofore not considered possible because of manufacturing limitations.
A further object of the present invention is to provide a screen plate structure wherein, for a given screen plate area, increased screening capacity is possible for increased throughput pulp screening rates, and wherein a variety of sizes and shapes are possible for the screen plate openings.
Another object of the present invention is to provide a screen plate forming process and a screen plate structure which can utilize relatively thin material to produce an aggressive profile for increasing the hydraulic capacity of the screen.
Yet another object of the present invention is to provide a modular screen plate structure which simplifies screen plate changing and which eliminates the need to change an entire screen plate when only a portion of the plate is damaged or worn.
A still further object of the present invention is to provide a screen plate structure and manufacturing process therefor which substantially reduce the manufacturing costs of a screen plate while improving the screening efficiency and throughput thereof.
Still another object of the present invention is to provide a screen plate structure and method of manufacturing which is more resistant to abrasive wear than heretofore known screen plates, thereby increasing the useful life of the screen plate when screening slurries containing highly abrasive contaminants.